FIGS. 5A, 5B, and 6 illustrate a known embodiment of a tandem master cylinder. These views are limited to cutaways of the primary piston.
Known master cylinder 300 consists of body 310 traversed by bore hole 320 along axis xx accommodating primary piston 330 controlled by a servobrake, not shown, or directly by action exerted on the brake pedal. Primary piston 330 delimits pressure chamber 340 connected to the primary brake circuit. Pressure chamber 340 is separated from supply chamber 350, consisting of a peripheral groove in body 310. On the forward side, supply chamber 350 is bordered by peripheral groove 351 accommodating resupply seal 353 and, in the rear, on the exterior side, supply chamber 350 is bordered by rear groove 352 accommodating seal or isolation seal 380. Primary piston 330 is formed of skirt 332 having an intermediary back wall 331 separating the hollow forward portion from the equally hollow rear portion. Back wall 331 receives, on rear face 333b, the thrust rod of the servobrake and, on its forward face 331a, back wall 333 presses against telescoping rod 370, formed of two parts 371, 372 and spring 373 cooperating with the secondary piston, which is not shown in FIGS. 5A, 5B, 6.
On the forward side, that is to say, on the side of pressure chamber 340 delimited by primary piston 330 in bore hole 320 of the master cylinder, skirt 332 is traversed by a crown of holes 333 that ensures communication between supply chamber 350 and the forward portion, inside the primary piston delimiting pressure chamber 340. This crown of holes 333 is realized in frustoconical groove 334 on the front of skirt 332 to facilitate the passage of brake fluid whenever primary piston 330 is in rest position and the brake system is operating in ESP mode. Crown of holes 333 is then found immediately below seal 353 and allows hydraulic fluid to flow from supply chamber 350 to pressure chamber 340.
FIG. 5A is a partial axial cutaway of known primary piston 330 installed in tandem master cylinder 300, which is only partly represented. This partial cutaway illustrates the particular shape of back wall 331, with its cavity, in rear face 331a receiving the thrust rod of the servobrake and front face 331b equipped with boss 335, which serves to center spring 373. FIG. 5B also illustrates frustoconical groove 334, whose taper is turned toward the front of primary piston 330 thereby relieving the forces exerted on seal 353, as shown in the partial cutaway of FIG. 5B.
Primary piston 330 is aluminum or an aluminum alloy and requires significant machining. Additionally, specific precautions must be taken for holes 333 in order to eliminate machining burrs. This is represented overall in a primary piston requiring relatively complex and costly machining and, subsequently, a costly tandem master cylinder.